All You Need to Know About Tire Sidewall Markings - Rubber Match

Here’s everything you ever wanted to know about those little numbers on the sidewall of tires but were afraid to ask

Tires are likely the most overlooked item on any hot rod. The problem is that they also have one of the biggest effects on a car’s performance. Most any average ride can vault to fast-car status by just tossing on some super-sticky race rubber. Even a car with a great suspension can be hobbled by hard tires that grip slightly better than butter. That’s why it’s important to put as much thought into choosing tires as you do on the more glamorous go-fast goodies. And while at first glance it may seem like tires are fairly straightforward, the truth is there’s a ton of science packed into every grooved ring of rubber. It may seem cool to run 40 treadwear autocross race slicks on your daily driver, but they will wear out quickly and heat cycle into hard and gripless junk before you know it.

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Show up to a speed-driving event on tires with too low a speed rating and you’ll end up a spectator instead of a driver.

Yep, shopping for tires is serious business, and to make the right choices you’ll need a little knowledge. Now, we could fill this book from cover to cover with tire info and still have more to say, so this time around we’ll just touch on the basics so you’ll know the terminology and what all that info stamped on the sidewall of every tire actually means. After all, knowledge is power, and who doesn’t want more power?

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Tread Design

Tread design combines a little art with a ton of science, as each tire manufacturer has their own ideas on what works best for a given driving segment.

The most common pattern is called symmetric. Symmetrical tires have the same pattern on both the inboard and outboard halves and it doesn’t care which direction it rotates. It makes rotating your tires easy, but they come up short in the performance department. In other words, they are great for your family sedan—not so hot for your Pro Touring Camaro.

Next up are tires with an asymmetric pattern. These combine great water dispersion with excellent dry grip since the designers can vary the tread pattern across the face of the tire. Usually they incorporate larger tread blocks on the outboard side for increased cornering stability on dry roads (which also reduces tread squirm and heat buildup on the outside shoulder). The inboard typically has smaller independent tread blocks for improved wet traction. A dead giveaway on these tires is that they will be stamped “INSIDE” on one side of the tire and “OUTSIDE” on the other.

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Directional pattern tires (also referred to as unidirectional) incorporate lateral grooves on both sides of the tire’s centerline that run in the same direction and create V- or wedge-shaped tread blocks. This really helps in the rain as the pattern “pumps” water more efficiently out from under the tire. These tires are designed to only spin in one direction and will be marked “LEFT” on one side of the tire and “RIGHT” on the other, or with directional arrows.

Lastly, there are asymmetric and directional patterned tires that combine the previous two designs. They're treated as directional in terms of rotation, and if you run different sized tires front and rear then each tire becomes location-specific and negates the possibility of rotating your tires.

What pattern works best? Post on any Internet message board and you’ll be deluged by “keyboard warriors” trying to push their point of view. If you plan on road racing, autocrossing, or carving up the nearest mountain road, then find out what others are running in the real world.

Now, let’s figure out what all the other data stamped into the side of your tire actually means.

Traction Grades

Asphalt g force

Concrete g force

AA

Above 0.54

0.41

A

Above 0.47

0.35

B

Above 0.38

0.26

C

Less than 0.38

0.26

Temperature Grade

Speeds (in mph)

A

Over 115

B

100 to 115

C

85 to 100

Speed Rating

mph

P

93

Q

99

R

106

S

112

T

118

U

124

V

130

Z

149+

W

168

Y

186

(Y)

186+

Load Ratings

Pounds

101

1,819

100

1,764

99

1,709

98

1,653

97

1,609

96

1,565

95

1,521

94

1,477

93

1,433

92

1,389

91

1,356

90

1,323

Trivia: a typical valve stem weighs about 1.2 ounces more than the aluminum that was bored out to make room for it.

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All You Need to Know About Tire Sidewall Markings - Rubber Match

Reading and understanding tire sizing is critical if you want to get the right rubber under your ride. It’s a bit tricky since the data is a mix of metric and standard numbers. In the above example the 305 represents the section width of the tire in millimeters; 305 mm to be precise. This is not the width of the tire, but instead it’s the measurement of the tire’s width from its outer sidewall to the widest part of its inner sidewall (not including ribs, raised letters, or other bumps). The measurement is taken with the tire mounted and inflated, so the rim width does affect the number. We were told that for every ½-inch change in rim width there’s a corresponding change in section width of 1⁄5-inch. Not much, but it does matter if you’re trying to shoehorn in the biggest tire possible. The second number is the aspect ratio. This is a measurement of the tire’s sidewall height as a percentage of the tire’s section width. So, if a tire has an aspect ratio 35 and a section width of 305, then we can calculate the

The Kumho Ecsta XS is an example of an asymmetric tread design. Other tires in this group include the Michelin Pilot Sport II, Falken Azenis, and Nitto Invo.

The Nitto NT05 uses a directional tread design. Other tires of this type are the BFG KDW, Nitto 555, Toyo R888, and Falken FK452

Reading and understanding tire sizing is critical if you want to get the right rubber under your ride. It’s a bit tricky since the data is a mix of metric and standard numbers. In the above example the 305 represents the section width of the tire in millimeters; 305 mm to be precise. This is not the width of the tire, but instead it’s the measurement of the tire’s width from its outer sidewall to the widest part of its inner sidewall (not including ribs, raised letters, or other bumps). The measurement is taken with the tire mounted and inflated, so the rim width does affect the number. We were told that for every ½-inch change in rim width there’s a corresponding change in section width of 1⁄5-inch. Not much, but it does matter if you’re trying to shoehorn in the biggest tire possible. The second number is the aspect ratio. This is a measurement of the tire’s sidewall height as a percentage of the tire’s section width. So, if a tire has an aspect ratio 35 and a section width of 305, then we can calculate the

Treadwear is something we could write an entire story on. The main problem is that it’s far less scientific than you would think and open to a lot of interpretation. In theory, the treadwear grade is a numeric number based on standards set forth by the U.S. Department of Transportation. The number describes how a tire company views the wear of a given tire they produce. A tire with a 200 treadwear should last twice as long (and have less lateral grip) as a tire with a 100 rating. The problem is that tire producers are under zero obligation to grade a tire based on the actual test results, except they can’t overstate a grade. So, they can call a 300 treadwear tire a 200, but they can’t refer to a 200 treadwear tire as a 280. It’s also common that for tires where treadwear isn’t important (i.e., race tires) are given treadwear ratings as low as zero. There’s also variance between different manufacturers, so a rating is really only somewhat accurate when comparing tires within a given brand. In short, a 220 trea

Over time, even when just stored on a rack, tires start to degrade. Given this, it’s important to know how fresh the tires are you’re buying. The subject of “expiration dates” is open to debate since different tires age at different rates based on their construction and how they are stored. We wouldn’t buy tires that have been sitting around for over four years. One or two years hanging out in a warehouse is preferable. Every tire should be stamped with a Tire Identification Number. Much of it is unimportant and refers to where the tire was made, size, and other data. The crucial part is the last four numbers (or three numbers if manufactured prior to 2000). The first two numbers indicate the week of manufacture and the last two are the year. So the above tire was churned out in the 17th week of 2009.

Many tires have cryptic red or yellow dots seemingly placed at random spots around the tire. These are actually there to help make balancing the tire and wheel assembly easier. The dots come in two flavors: red and yellow. The red dot is considered the “high spot” of the tire. It’s complicated and involves a lot of physics, but in short you would mount this “high spot” with a wheel’s (usually steel) “low spot.” This is typically marked on the wheel with a dimple. On an aluminum wheel there likely won’t be a mark, so the red dot is put opposite the valve stem to help counteract the centrifugal force effect of the valve stem’s greater weight. If the tire has a yellow dot, then always mount that opposite the valve stem. The Yellow dot indicates a tire’s “light” static balance point. If a tire has both a yellow and red dot, ignore the yellow and concentrate on the red.

Speed ratings exist to help match the performance of the tire with the performance of the car. They are established in kph then converted to mph (which explains the wonky increments). The ratings are based on lab tests on new, properly inflated tires, so damage, age, and wear can lower the tire’s effective speed rating. Early on, the speed rating was embedded within the tire size e.g., 235/50/VR16), but after 1991 the rating had to be shown in a separate portion of the tire’s description (e.g., 235/50R16 89V, with V being the speed rating).

When Z-rated tires first hit the market nobody thought they would need anything higher than 149 mph. They also never specified how far over 149 mph the tires could go. As cars kept getting faster, the industry introduced W and Y speed ratings. When a Z rating shows up in the tire designation (e.g., 235/50ZR16 91W) the Z signifies it’s capable of speeds over 149 and the W indicates a top speed of 168. If the tire is rated Y, then it’s good to go up to 186 mph. If the Y rating is in parentheses (with the load rating as shown above) then the top speed has been tested in excess of 186 mph. We clear?

So, how does a super-sticky track tire with a 00 treadwear rating end up with a Traction grade of B? The answer is that the traction grade is based on a tire’s straight-line WET coefficient of traction as it skids across a specific test surface. It has absolutely nothing to do with dry cornering, dry braking, hydroplaning, or even wet cornering. The temperature grade indicates how a tire generates, dissipates, and resists the effects of heat. Obviously, if a tire can’t handle heat buildup, then it won’t be able to handle high speeds. Any tire that has a higher speed rating will most assuredly have a temperature grade of A.

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